The present invention relates to audio signal processing, and, in particular, to a reduction of comb filter artifacts in a multi-channel downmix with adaptive phase alignment.
Several multi-channel sound formats have been employed, from the 5.1 surround that is typical to the movie sound tracks, to the more extensive 3D surround formats. In some scenarios it is necessitated to convey the sound content over a lesser number of loudspeakers.
Furthermore, in recent low-bitrate audio coding methods, such as described in J. Breebaart, S. van de Par, A. Kohlrausch, and E. Schuijers, “Parametric coding of stereoaudio,” EURASIP Journal on Applied Signal Processing, vol. 2005, pp. 1305-1322, 2005 and J. Herre, K. Kjörling, J. Breebaart, C. Faller, S. Disch, H. Purnhagen, J. Koppens, J. Hilpert, J. Röden, W. Oomen, K. Linzmeier, and K. S. Chong, “MPEG Surround—The ISO/MPEG standard for efficient and compatible multichannel audio coding,” J. Audio Eng. Soc, vol. 56, no. 11, pp. 932-955, 2008, the higher number of channels is transmitted as a set of downmix signals and spatial side information with which a multi-channel signal with the original channel configuration is recovered. These use cases motivate the development of downmix methods that preserve well the sound quality.
The simplest downmix method is the channel summation using a static downmix matrix. However, if the input channels contain sounds that are coherent but not aligned in time, the downmix signal is likely to attain perceivable spectral bias, such as the characteristics of a comb filter.
In J. Breebaart and C. Faller, “Spatial audio processing: MPEG Surround and other applications”. Wiley-Interscience, 2008 a method of phase alignment of two input signals is described, which adjusted the phases of the input channels based on the estimated inter-channel phase difference parameter (ICPD) in frequency bands. The solution provides similar basic functionality as the method proposed in this paper, but is not applicable for downmix more than two inter-dependent channels.
In WO 2012/006770, PCT/CN2010/075107 (Huawei, Faller, Lang, Xu) a phase alignment processing is described for a two to one channel (stereo to mono) case. The processing is not directly applicable for multichannel audio.
In Wu et al, “Parametric Stereo Coding Scheme with a new Downmix Method and whole Band Inter Channel Time/Phase Differences”, Proceedings of the ICASSP, 2013a method is described that uses whole-band inter-channel phase difference for stereo downmix. The phase of the mono signal is set to the phase difference between the left channel and the overall phase difference. Again, the method is just applicable for stereo to mono downmix. More than two inter-dependent channels cannot be downmixed with this method.